Wind Tech: Record rotor size boosts power rating

WORLDWIDE: With the 7MW V164-7.0MW offshore turbine, Vestas has introduced a higher power rating compared with several 6MW+ class main competitors and a record-size rotor diameter. One uncommon aspect of the machine, both for Vestas and the industry at large, is the use of a medium speed, geared drive system.

Speaking about the design process behind V164, Vestas Technology president Finn Stromm Madsen says it was driven by two aims beyond lowering the machine's all-important cost of energy. These were choice of the drive system and its serviceability. Madsen says: "Initially we followed two parallel drive system development tracks, direct drive and geared. A key design team question was to determine the main factors causing wind turbine downtime. Our in-house studies supplemented by third-party reports from 2010 show that faulty electrical components cause more failures than gearboxes."

The second finding was that other mechanical components such as fail-safe brake and blade bearings also cause more downtime than gearboxes.

Direct-drive generators contain four times as many electrical components as medium-speed geared drive systems, explains Madsen. "For Vestas, the main conclusion was that wind turbine reliability discussions should not focus on gearboxes. We had also decided that no technology solution should create high and unwanted dependence upon rare-earth elements used for the magnets in permanent-magnet generators."

The main outcome of the comparative drive system study favoured a three-stage medium-speed drive system, which in the specific V164-7.0MW configuration steps up 10-12 rotor revolutions to 400rpm rated generator speed. The drive train concept itself consists of a main shaft supported in two main bearings, and as an assembly incorporated into a single-cast housing. It is further connected to the gearbox and generator via a low-speed coupling.

The 3.3kV Vestas-design permanent-magnet type generator is liquid-cooled and based upon similar design principles to the smaller V112-3.0MW generator. The V164-7.0MW converter and 33-34kV transformer will be located in the tower base, ensuring easy access. A 66kV option exists.

Vestas will not offer standard towers for the V164-7.0MW, but has said the typical hub height will be around 107 metres.

The new 80-metre slender blade follows a similar design to that used on the 54.6 metre V112-3.0 blade. Vestas has used carbon fibres to enhance stiffness and as a mass-optimising feature in the central blade spar. The V164-7.0MW blade mass is 35 tonnes. The combined mass of the nacelle plus separate transformer is about 290 tonnes, whereas the rotor mass - hub plus blades - is approximately 210 tonnes. The nacelle is 24 metres long, by 12 metres and 7.5 metres.

Despite its plans to launch a next-generation offshore turbine, Nordex is a relative newcomer to the sector. So far, its only two marine-specific machines are an adapted 2.3MW N90/2300 and a 2.5MW N90/2500 prototype, installed in 2003 and 2006 respectively.

From Nordex's point of view, its first offshore-specific turbine, the 6MW N150/6000, is notable as earmarking its first switch to direct drive. The company says this can result in higher reliability and thus potentially better availability due to significantly fewer components. The turbine also has 55 tonne/megawatt top head mass, comparing favourably with 70-90 tonne/megawatt quoted for current "relatively heavy and expensive" second-generation 5MW turbines.

Besides an approximately 300-tonne top head mass for the nacelle and rotor, another distinctive design feature of the N150/6000 is a permanent-magnet type liquid-cooled generator located behind the tower.

Explaining the drive system layout preferences, Nordex chief technical officer Eberhard Voss says the main considerations, besides low cost of energy, were serviceability and easy exchange of main components.

Speaking about the design, he adds: "The rotor and generator are fitted to separate rotating shafts, each with their own bearing sets. These two shafts in turn are interconnected by a hollow torque shaft, which results in a substantial mass saving compared with a rear-mounted generator solution but with a single, long drive shaft."

He adds that the turbine's outer rotor generator design, with the rotating part turning around the stationary part, offers higher torque and better efficiency compared with inner rotor equivalents. Nordex has long experience with liquid-cooled generators, dating back to the 1.3MW N60 in the 1990s.

Discussing the blades, Voss says Nordex has succeeded in retaining a 10.5-tonne mass for three successive blade generations, each five metres longer than the last, to match a corresponding ten-metre increase in rotor diameter.

"For the N80, N90, and N100 blades, glass-fibre reinforced epoxy composite material is applied," he says. "We incorporated carbon fibres for the first time in the 57-metre slender blades for the N117/2400 and will do this again in the much longer N150/6000 rotor blades. Carbon fibres as a key characteristic enable a combination of high stiffness and slender blade design."

Some competitors have introduced cyclic pitching in their latest large-diameter turbine models, a technology whereby the blade angle is continuously adjusted during each rotor revolution as a load-reducing control and optimising measure. However, Voss is not convinced about the overall benefits associated with this.

He says: "Rotor blade pitch systems typically operate at 0.1-0.2 degrees per second, which is too slow for adequately meeting advanced load-control needs. In addition, frequent pitching actions put high strain upon the pitch system and this could result in accelerated wear and increased failure risk."

Nordex plans to install an onshore prototype of the N150/6000 in 2012, followed by an offshore prototype in 2013 - milestones in a years-long offshore strategy. Nordex launched its offshore division, Nordex Offshore, in 2010. To fast-track its entry to the offshore wind market, it then acquired a 40% share in major German wind farm Arcadis Ost 1. Nordex plans to deliver up to 70 turbines for the offshore project in 2014-15.

German gearbox supplier Winergy has introduced a new compact medium-speed drive system that is suited for geared offshore and onshore wind turbines with power ratings of 6-7MW. A prototype of the technology, dubbed HybridDrive, will, however, be first fitted into a new 3MW Fuhrlander FL 3000 IEC WC IIa wind turbine model with rotor diameter of 120 metres. The prototype is expected to be completed by next April.

A HybridDrive consists of a two-stage planetary gear system and a permanent magnet-type synchronous generator integrated into a single product. A power converter can supplement HybridDrive, as Winergy is likely the world's sole fully integrated wind turbine drive system supplier able to design and manufacture gearboxes, generators and converters.

Development of the technology marks cooperation between German companies Winergy, Fuhrlander and wind turbine development consultancy W2E Wind to Energy. W2E earlier developed a smaller 2.5MW Fuhrlander FL 2500 turbine model, fitted with a flanged gearbox and a non-integrated, double-fed induction generator or DFIG. Both turbine models feature a patented Larus Compact drive train system, which as a main design feature transmits "pure torque" while preventing harmful rotor-bending moments from entering the gearbox.

A key focus of the HybridDrive's product development was compact design. A direct flange connection of the two-stage gearbox to the generator eliminates the need for an intermediate shaft. According to Winergy business development representative Tobias Hang, this enables the drive train length to be shortened by about 35% compared with a conventional non-integrated geared drive system of similar power rating.

An additional goal was to achieve maximum operational efficiency under both partial and peak load. Winergy claims that its total system peak efficiency of over 94% is among the best of all existing drive technologies. The HybridDrive generation only requires about 20% of the rare-earth element needed in direct-drive generators of the same power rating, Hang says.

Overall, the SWT-2.3-113 shares a similar concept to its earlier sister product, the SWT-3.0-101. Elaborating on the new design, Siemens Wind Power chief technology officer Henrik Stiesdal says: "Due to the larger rotor diameter, we reduced rated rotor speed from 16rpm to 13rpm, but the generator torque level remains unchanged. The reduced power enables us to apply the same generator to both sister models, and with the larger rotor we get much more energy at low and medium wind sites."

So far, five SWT-3.0-101 turbines have been installed, including two prototypes in Denmark and Norway. One of the 15 turbines currently being built is a "final" series model. This incorporates optimisations such as a single turbine control cabinet instead of two, and a new onboard crane.

It is possible to walk through the hollow generator for access to the rotor hub, while the power converter and medium-voltage transformer are located in the tower foot. In some other concepts the converter and power converter are located in the nacelle.

An alternative is to locate a rectifier in the nacelle, feed DC power down the tower, and put the inverter part and transformer in the tower foot. When the rectifier and inverter are combined into one assembly it is called a converter.

The SWT-2.3-113 Quantum Blade is fitted with a new generation of in-house developed blades. Siemens' IntegralBlade manufacturing process casts the glass-fibre reinforced epoxy blades in one piece, eliminating the leading edge and trailing edge glue joints traditionally considered to be weak points.

Stiesdal says the company has consistently reduced mass in rotor blades developed in-house, starting in 1998 with the first 30-metre long B30 blade for a 1.3MW former Bonus turbine. Specific mass, in terms of kilogram/metre, of later rotor blade types - the B40, B45 and B55 - gradually dropped.

Stiesdal explains: "Our first new-generation B49 and B58 slender blades were developed for the 2.3MW and 3.6MW standard geared turbines. The single most important benefit of these load-reducing slender blades is that - despite significant growth in rotor swept area and therefore yields - the dynamic loads did not significantly increase."

With the new SWT-2.3-113 Quantum Blade for the direct-drive platform, significant specific-mass gains have again been made without having to apply carbon fibres. The model is expected to become commercially available in 2012.

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